How can I add tools like GCC on EOS?

Endless doesn't ship with tools like gcc in the base OS, but you can add them to run/build apps inside a Sandbox using Flatpak. http://flatpak.org/developer.html has detailed developer instructions, but these commands will get you a shell where you can run GCC:

You may be interested in Builder (an IDE for GNOME) and other development tools which are available for download via the app center. (If you're interested in working on components of a Flatpak runtime, you may find Flapjack useful).